Notes

If you need to serialize an object with a PickledObjectField for transmission
to the browser, you may need to subclass the field and override the
value_to_string() method. Currently pickle fields are serialized as
base64-encoded pickles, which allows reliable deserialization, but such a
format is not convenient for parsing in the browser. By overriding
value_to_string() you can choose a more convenient serialization format.

Original notes

Here are the notes by taavi223, the original author:

Incredibly useful for storing just about anything in the database (provided it
is Pickle-able, of course) when there isn’t a ‘proper’ field for the job.

PickledObjectField is database-agnostic, and should work with any database
backend you can throw at it. You can pass in any Python object and it will
automagically be converted behind the scenes. You never have to manually pickle
or unpickle anything. Also works fine when querying; supports exact, in, and
isnull lookups. It should be noted, however, that calling QuerySet.values()
will only return the encoded data, not the original Python object.

This PickledObjectField has a few improvements over the one in snippet #513.

This one solves the DjangoUnicodeDecodeError problem when saving an object
containing non-ASCII data by base64 encoding the pickled output stream. This
ensures that all stored data is ASCII, eliminating the problem.

PickledObjectField will now optionally use zlib to compress (and uncompress)
pickled objects on the fly. This can be set per-field using the keyword
argument “compress=True”. For most items this is probably not worth the small
performance penalty, but for Models with larger objects, it can be a real space
saver.

You can also now specify the pickle protocol per-field, using the protocol
keyword argument. The default of 2 should always work, unless you are trying to
access the data from outside of the Django ORM.

Worked around a rare issue when using the cPickle and performing lookups of
complex data types. In short, cPickle would sometimes output different streams
for the same object depending on how it was referenced. This of course could
cause lookups for complex objects to fail, even when a matching object exists.
See the docstrings and tests for more information.

You can now use the isnull lookup and have it function as expected. A
consequence of this is that by default, PickledObjectField has null=True set
(you can of course pass null=False if you want to change that). If null=False
is set (the default for fields), then you wouldn’t be able to store a Python
None value, since None values aren’t pickled or encoded (this in turn is what
makes the isnull lookup possible).

You can now pass in an object as the default argument for the field without it
being converted to a unicode string first. If you pass in a callable though,
the field will still call it. It will not try to pickle and encode it.

You can manually import dbsafe_encode and dbsafe_decode from fields.py if you
want to encode and decode objects yourself. This is mostly useful for decoding
values returned from calling QuerySet.values(), which are still encoded
strings.

Note: If you are trying to store other django models in the PickledObjectField,
please see the comments for a discussion on the problems associated with doing
that. The easy solution is to put django models into a list or tuple before
assigning them to the PickledObjectField.

Update 9/2/09: Fixed the value_to_string method so that serialization should
now work as expected. Also added deepcopy back into dbsafe_encode, fixing #4
above, since deepcopy had somehow managed to remove itself. This means that
lookups should once again work as expected in all situations. Also made the
field editable=False by default (which I swear I already did once before!)
since it is never a good idea to have a PickledObjectField be user editable.

Changes

Changes in version 0.3.1

Favor the built in json module (thanks to Simon Charette).

Changes in version 0.3.0

Python 3 support (thanks to Rafal Stozek).

Changes in version 0.2.0

Allow pickling of subclasses of django.db.models.Model (thanks to Simon
Charette).